What you're not taking into account is that computers are increasingly used to help design and verify the next generation of chips.

That is taken into account here: "Machines are already heavily involved in the design of other machines. No-one could design a modern CPU without the use of computers. No one could build one without the help of sophisticated machinery."

It does mean we have no way to make a better prediction than that. It means that is the prediction on which rationalists should base their plans.

Not according to Robin Hanson. I'm not sure how much stock I put in an extrapolation from four data points, some of which are millions of years old - but the conclusion seems plausible: we have independent evidence that something big is coming, because we can see it on the horizon.